Issue connecting SPA3102 FXO port to paging interface

Question

Hello,
I was referred to this community after a call to small business support, we were unable to come to a solution.
SPA3102 v5.2.13 FW
Viking Electronics PI-1 Paging Interface
We were told from Viking that the interface only supports an FXO port, all the connections were verified with their support and we're able to page by connecting an analog phone straight to the interface. Plugging into the FXS port on the SPA3102 also results in a dial tone playing over the paging system.
All settings on the SPA3102 are default minus SIP registration information, we see the unit as registered within the web interface as well as within our Asterisk CLI.
Trying to ring the extension results in constant ringing if voicemail is disabled, and straight to voicemail if enabled. Within the Asterisk CLI we see a SIP response 503 "Service Unavailable."
I can get the SPA3102 to respond by lowering the "Line In Use Voltage" down from its default value of 30. However all I hear is a hum and I'm not able to speak over the paging system.
Any insight into this issue would be greatly appreciated, we believe it's just settings on the SPA3102 unit itself that need to be changed.
Thanks!

Answer 1

Looking at the Viking Pl-1 web page installation instructions, I think the key setting on the SPA3102 would be to set the voip-to-pstn gateway dial plan to NONE.  The "hum" you get when you lower the Line-In-Use setting sounds like the dial tone you would get with the default dial plan setting.
I would try the following:
Viking Pl-1: 
Talk Battery Switch (PT): On
Audio In: Cabled to FXO port of SPA3102
SPA3102:
Interface Analog Phone attached to SPA3102:
Line 1 Tab
Line Enable: YES
Dial Plan: (S0<:@gw0>)
PSTN Line Tab
Line Enable: Yes
VoIP-to-PSTN Gateway Enable: Yes
One Stage Dialing: Yes
Line 1 VoIP Caller DP: NONE
VoIP Answer Delay: 0
Line In Use Voltage: xx
The Line In Use Voltage needs to be lower than the Talk Battery voltage supplied by the Viking Pl-1.  Usually it is set about half way between the on-hook and off-hook voltage level.  Measure the FXO (PSTN Line) on-hook voltage by reading it on the SPA3102 INFO Tab.  Set the Line In Use Voltage substantially below the on-hook talk voltage.
If you lift the phone with the above settings you should be connected to the paging system.
Interface to an asterisk pbx system:
PSTN Line Tab
Setup Registration on PSTN Line Tab to Asterisk system. 
Register: Yes
Proxy: xxx
UserID: xxx
Password: xxx
VoIP-to-PSTN Gateway Enable: Yes
VoIP Caller Auth Method: None
One Stage Dialing: Yes
VoIP Caller Default DP: NONE
VoIP Answer Delay: 0
Line In Use Voltage: same comments as above
If you call the extension on the asterisk PBX you should be attached to the Viking unit.